Bonding agent and lithium-ion battery thereof

ABSTRACT

The present disclosure relates to the field of lithium-ion battery materials, and specifically relates to a lithium-ion battery bonding agent and a lithium-ion battery comprising said bonding agent. The bonding agent is a polymer comprising structural units represented by Formula I, Formula II, Formula III, and Formula IV, and has a number average molecular weight of 500,000 to 1.2 million. The present disclosure further relates to a lithium-ion battery, comprising a positive film, a negative film, a separator, and an electrolyte, the positive film comprising the bonding agent according to the present disclosure. The bonding agent according to the present disclosure can significantly improve the flexibility of a positive film, avoid processability and battery performance issues caused by the electrode film being too brittle, and help improve the compact density and battery energy density.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Chinese Patent Application No.201610033956.0, entitled “BONDING AGENT AND LITHIUM-ION BATTERY THEREOF”and filed on Jan. 19, 2016 in the State Intellectual Property Office ofthe People's Republic of China (PRC) (SIPO), the disclosure of which isexpressly incorporated by reference herein in its entirety.

BACKGROUND

Field

The present disclosure relates generally to the field of lithium-ionbatteries, and more particularly, to a lithium-ion battery bonding agentand a lithium-ion battery comprising said bonding agent.

Background

With advantages such as high energy density, high working voltage, andlong cycle life, lithium-ion batteries have been extensively used oncivilian apparatuses such as mobile phones, laptops, and digitalproducts, as well as high-tech equipment such as unmanned aerialvehicles, space flight, and satellites. Lithium-ion batteries mainlycomprise a positive film, a negative film, a separator and anelectrolyte. The negative film typically uses natural graphite orartificial graphite as an active substance, sodium carboxymethylcellulose (CMC) as a dispersing agent, and styrene-butadiene rubber(SBR) as a bonding agent. In the positive film, on the other hand,lithium cobalt oxide (LiCoO₂) is commonly used as an active substance,conductive carbon as a conductive agent, and polyvinylidene difluoride(PVdF) as a bonding agent.

When fabricating a lithium-ion battery, firstly, coat positive andnegative electrode pastes on corresponding current collectors, dry andthen subject to cold pressing, and then perform subsequent processes.After cold pressing, graphite, CMC, and SBR in the negative electrode(anode) are all soft materials themselves, thus the electrode film isrelatively soft. As for the positive electrode (cathode), however,LiCoO₂ and PVdF have relatively high hardness, and PVdF has a strongcrystallinity, making it easy to have the issue that the electrode filmbecomes brittle after cold pressing, and may lead to problems that theactive substance cracks and falls off in the subsequent strip divisionand sheet cutting, and that the electrode film breaks in the windingprocess, which detrimentally affect the battery production andperformance.

To improve the flexibility of a positive film after cold pressing, amethod of modification by copolymerization is typically adopted on PVdF.Homopolymer PVdF has a strong crystallinity, and the material itself hashigh strength and hardness, and the introduction of other structuralunits into PVdF by means of modification by copolymerization may reducethe degree of order of the PVdF molecular structure, lowercrystallinity, and may improve the ductility and elongation at break ofPVdF. When the modified PVdF is used as a bonding agent for a positivefilm, the flexibility of the electrode film after cold pressing can beimproved.

For performing modification by copolymerization on PVdF, a commonly usedcopolymerization monomer is hexafluoropropylene, and the obtainedPVdF-HFP has a significantly lowered crystallinity and improvedelongation at break relative to homopolymer PVdF. When PVdF-HFP is usedas a bonding agent for a positive film, the flexibility of the electrodefilm is improved, which helps to improve the compact density, thusimproving battery energy density. However, the bonding force of PVdF-HFPis significantly reduced relative to copolymer PVdF. During themanufacture process, the positive electrode active substance could falloff as a result of the low bonding force. When an electrolyte isinjected into the battery, PVdF-HFP swells greater than that ofhomopolymer PVdF does, the bonding force of the positive film decreasesby a greater degree, leading to the problem of the active substancefalling off after formation and aging and during the battery use, andconsequently leading to significantly decreased performance or failureof the battery.

The present disclosure is hereby proposed in view of the drawbacks ofthe traditional technologies.

SUMMARY

A first objective of the present disclosure is to provide a bondingagent.

A second objective of the present disclosure is to provide a lithium-ionbattery comprising said bonding agent.

To attain the objectives of the present disclosure, the followingtechnical solution is employed:

The present disclosure relates to a bonding agent, said bonding agentbeing a polymer comprising structural units represented by Formula I,Formula II, Formula III, and Formula IV:

Wherein, each of R₁, R₂, R₃, and R₄ is independently selected from thegroup consisting of hydrogen, and C₁₋₈ straight-chain or branched alkylgroups substituted by a substituting group or not substituted by asubstituting group; each of R₅, R₆, and R₇ is independently selectedfrom the group consisting of hydrogen, and C₁₋₆ straight-chain orbranched alkyl groups substituted by a substituting group or notsubstituted by a substituting group; R₈ is selected from C₁₋₁₅ alkylgroups substituted by a substituting group or not substituted by asubstituting group; each of R₉, R₁₀, and R₁₁ is independently selectedfrom the group consisting of hydrogen, and C₁₋₆ straight-chain orbranched alkyl groups substituted by a substituting group or notsubstituted by a substituting group.

The substituting group is selected from halogens. And each of n₁, n₂, n₃and n₄ is independently an integer greater than 0.

In one aspect, the molar percent of structural units represented byFormula I in the bonding agent is 50% to 90%, and in a further aspect,60 to 75%. In one aspect, the molar percent of structural unitsrepresented by Formula II in the bonding agent is 0.1% to 20%, and in afurther aspect, 5% to 10%. In one aspect, the molar percent ofstructural units represented by Formula III in the bonding agent is 1%to 25%, and in a further aspect, 10% to 25%. In one aspect, the molarpercent of structural units represented by Formula IV in the bondingagent is 0.1% to 10%, and in a further aspect, 3% to 5%.

In one aspect, in Formula II, each of R₁, R₂, R₃, and R₄ isindependently selected from the group consisting of hydrogen, and C₁₋₆straight-chain or branched alkyl groups. In one aspect, each of R₁, R₂,R₃, and R₄ is independently selected from the group consisting ofhydrogen and C₁₋₃ alkyl groups.

In one aspect, in Formula III, each of R₅, R₆, and R₇ is independentlyselected from the group consisting of hydrogen and C₁₋₃ alkyl groups. R₈is selected from C₁₋₁₂ alkyl groups substituted by a substituting groupor not substituted by a substituting group.

In one aspect, in Formula III, each of R₅, R₆, and R₇ is independentlyselected from the group consisting of hydrogen, methyl, andtrifluoromethyl. R₈ is selected from the group consisting of methyl,ethyl, n-butyl, isobutyl, sec-butyl, t-butyl, n-propyl, cyclohexyl, C₁₂alkyl, 2-ethylhexyl, isobornyl, trifluoroethyl, and trifluoromethyl.

In one aspect, in Formula IV, each of R₉, R₁₀, and R₁₁ is independentlyselected from the group consisting of hydrogen and C₁₋₃ alkyl groups. Inone aspect, each of R₉, R₁₀, and R₁₁ is independently selected from thegroup consisting of hydrogen, methyl, and trifluoromethyl.

In one aspect, the number average molecular weight of the bonding agentis 500,000 to 1.2 million.

The present disclosure further relates to a lithium-ion battery,comprising a positive film, a negative film, a separator, and anelectrolyte. At least one of the positive film, the negative film, andthe separator comprises the bonding agent according to the presentdisclosure.

In one aspect, the lithium-ion battery comprises the bonding agent inthe positive film.

In one aspect, the positive film comprises a positive electrode currentcollector and a positive electrode active substance layer, and thepercent by weight of the bonding agent in the positive electrode activesubstance layer is 1.0% to 5.0%.

The present disclosure has the following advantageous effects:

Using a copolymerization method, the present disclosure obtainsvinylidene difluoride-alkyl unit-acrylate-acrylic acid copolymer(PVdF-Ac). By controlling monomer ratios in the polymerization process,the contents of alkyl unit, acrylate, and acrylic acid structural unitin PVdF-Ac can be adjusted. The main structural unit in PVdF-Ac is PVdF,which has excellent anti-oxidation and electrochemical stability.

Compared with homopolymer PVdF that has a similar molecular weight,alkyl unit, acrylate, and acrylic acid are copolymerized in PVdF-Ac andtherefore, the regularity of the molecular chain is reduced, thematerial has a lowered crystallinity, which is reflected by a softmaterial and high elongation at break. When alkyl units arecopolymerized, the flexibility and tensile strength of the material haveboth been improved, which helps to improve the compact density andenergy density of the battery. By introducing a structural unit ofacrylate, the regularity of the main PVdF-Ac chain is further reduced,the crystallinity is lowered, and the acrylate structural unit isrelatively soft, which directly results in the elongation at break ofthe PVdF-Ac adhesive film being greater than that of homopolymer PVdF.

When used on a positive electrode of a lithium-ion battery, given thesame compact density, the electrode film according to the presentdisclosure has a significantly superior flexibility than homopolymerPVdF does, as well as excellent processability. PVdF-Ac has a lowcrystallinity and swells greatly in an electrolyte, but acrylate in thestructure of PVdF-Ac has a high bonding force and good affinity withaluminum foil. The —COOH unit in acrylic acid monomers can have a stronghydrogen bond action with aluminum foil. As a result, although PVdF-Acswells greater in an electrolyte than homopolymer PVdF does, its bondingforce with an active substance and aluminum foil is greater than that ofhomopolymer PVdF. Moreover, the electrode film has better flexibilityand excellent processability. After swelling through immersion in anelectrolyte, the acrylate units have very good ion-transferringperformance, and the performance is also better than homopolymer PVdFwhen a lithium-ion battery is made.

The use of PVdF-Ac according to the present disclosure as a bondingagent for a positive electrode of a lithium-ion battery cansignificantly improve flexibility of the positive film, avoidmanufacture and battery performance issues caused by the electrode filmbeing too brittle, and help improve the compact density and energydensity of the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating comparison curves of breaking strengthand elongation at break of adhesive films of PVdF-Ac and homopolymerPVdF.

FIG. 2 is a diagram illustrating a photo of an electrode film withhomopolymer PVdF as a bonding agent.

FIG. 3 is a diagram illustrating a photo of an electrode film withcopolymer PVdF-Ac as a bonding agent.

DETAILED DESCRIPTION

The present disclosure and the advantageous effects of certainconfigurations will be further described in detail below with referenceto the accompanying drawings and specific embodiments. It should beunderstood that these embodiments are only used to describe the presentdisclosure and are not used to limit the scope of the presentdisclosure.

The present disclosure provides a positive electrode bonding agent, aswell as a positive film and a lithium-ion battery fabricated with saidbonding agent. Without detrimentally impacting the performance of thelithium-ion battery, the present disclosure significantly improves thebonding force of the positive film of the lithium-ion battery, andimproves the flexibility of the positive film and processabilitythereof.

The technical solution employed by the present disclosure is: comparedwith homopolymer PVdF, the present disclosure copolymerizes alkyl,acrylate, and acrylic acid structural units, and also introduces alkylunits, which can improve mechanical properties of said copolymer, suchas tensile strength and flexibility. The acrylate monomer can furtherimprove flexibility of said copolymer, increase swelling, and improveionic conductivity. Since the positive electrode current collector of alithium-ion battery is aluminum foil, the surface of the positiveelectrode current collector is typically an oxidizing layer, whichcontaining relatively more oxygen-containing groups. By introducingacrylic acid monomers, —COOH has a hydrogen bond action withoxygen-containing groups on the surface of aluminum foil, whichsignificantly improves the bonding force of the electrode film. When thebonding agent according to the present disclosure is used to prepare apositive film, the positive film has a high bonding force, goodflexibility, and excellent processability. After being immersed in anelectrolyte, it has relatively good ion-transferring performance, whichcan improve the performance of the lithium-ion battery.

The structural formula of the bonding agent according to the presentdisclosure is shown by Formula V:

Wherein, in one aspect, a=50% to 90%, b=0.1% to 20%, c=1% to 25%, d=0.1%to 10%; and in a further aspect, a=60 to 75%, b=5% to 10%, c=10% to 25%,d=3 to 5%.

In one aspect, each of R₁, R₂, R₃, and R₄ is independently selected fromthe group consisting of hydrogen, and C₁₋₈ straight-chain or branchedalkyl groups substituted by a substituting group or not substituted by asubstituting group; further, in one aspect, each of R₁, R₂, R₃, and R₄is independently selected from the group consisting of hydrogen, andC₁₋₆ straight-chain or branched alkyl groups; yet further, in oneaspect, each of R₁, R₂, R₃, and R₄ is independently selected from thegroup consisting of hydrogen and C₁₋₃ alkyl groups.

In one aspect, each of R₅, R₆, and R₇ is independently selected from thegroup consisting of hydrogen, and C₁₋₆ straight-chain or branched alkylgroups substituted by a substituting group or not substituted by asubstituting group; further, in one aspect, each of R₅, R₆, and R₇ isindependently selected from the group consisting of hydrogen and C₁₋₃alkyl groups; yet further, in one aspect, each of R₅, R₆, and R₇ isindependently selected from the group consisting of hydrogen, methyl,and trifluoromethyl.

In one aspect, R₈ is selected from C₁₋₁₅ alkyl groups substituted by asubstituting group or not substituted by a substituting group; further,in one aspect, R₈ is selected from C₁₋₁₂ alkyl groups substituted by asubstituting group or not substituted by a substituting group; yetfurther, in one aspect, R₈ is selected from the group consisting ofmethyl, ethyl, n-butyl, isobutyl, sec-butyl, t-butyl, n-propyl,cyclohexyl, C₁₂ alkyl, 2-ethylhexyl, isobornyl, trifluoroethyl, andtrifluoromethyl.

In one aspect, each of R₉, R₁₀, and R₁₁ is independently selected fromthe group consisting of hydrogen, and C₁₋₆ straight-chain or branchedalkyl groups substituted by a substituting group or not substituted by asubstituting group; further, in one aspect, each of R₉, R₁₀, and R₁₁ isindependently selected from the group consisting of hydrogen and C₁₋₃alkyl groups; yet further, in one aspect, each of R₉, R₁₀, and R₁₁ isindependently selected from the group consisting of hydrogen, methyl,and trifluoromethyl.

The substituting group is selected from halogens. In one aspect, thehalogen may be F, Cl, or Br.

In one aspect, the number average molecular weight of the bonding agentis 500,000 to 1.2 million.

A preferred upper limit of number of carbon atoms in the alkyl groupsdescribed above is sequentially 15, 12, 10, 8, 6, 4 and 3. For example,if the upper limit of number of carbon atoms is 12, the range of numberof carbon atoms in the alkyl groups is 1 to 12; the preferable number ofcarbon atoms in the alkyl groups is 1 to 6, and the further preferablenumber of carbon atoms in the alkyl groups 1 to 3. The alkyl group maybe a linear alkyl group or a cycloalkyl group. The linear alkyl groupcomprises straight-chain alkyl groups and branched alkyl groups. Thecycloalkyl group is a saturated alkyl group that contains an alicyclicring structure. The alicyclic ring may or may not contain a substitutinggroup.

The above C₁₋₁₅ alkyl groups include, but are not limited to: —CH₃,—CH₂CH₃, —(CH₂)₂CH₃, —CH(CH₃)₂, —(CH₂)₃CH₃, —CH₂CH(CH₃)₂,—CH(CH₃)CH₂CH₃, —CH₂CH(CH₃)₂, —C(CH₃)₃, —(CH₂)₄CH₃, —CH₂CH₂CH(CH₃)₂,—CH(CH₃)CH₂CH₂CH₃, —CH₂CH(CH₃)CH₂CH₃, —CH₂C(CH₃)₃, n-heptyl,cyclopropyl, cyclohexyl, n-octyl, 2-ethylhexyl, n-C₁₀ alkyl groups,n-C₁₂ alkyl groups, n-C₁₅ alkyl groups, and isobornyl.

Wherein, the maximum value of a may be 90%, 85%, 80%, 75%, 70%; and theminimum value of a may be 50%, 53%, 55%, 60%, etc. For example, therange of values for a may be 55% to 85%, 60% to 85%, etc. The maximumvalue of b may be 20%, 18%, 15%, 12%, 10%, etc.; and the minimum valueof b may be 0.1%, 1%, 2%, 4%, etc. For example, the range of values forb may be 1% to 20%, 2 to 15%, 5 to 15%, etc. The maximum value of c maybe 25%, 22%, 20%, etc.; and the minimum value of c may be 1%, 2%, 5%,8%, 10%, etc. For example, the range of values for c may be 2% to 25%, 5to 25%, 5 to 20%, etc.

The present disclosure further relates to a preparation method for saidbonding agent: preparing said bonding agent by means of emulsionpolymerization on PVdF monomer, alkene monomer unit, acrylate monomer,and acrylic acid monomer.

The present disclosure further relates to a lithium-ion battery,comprising a positive film, a negative film, a separator, and anelectrolyte, at least one of the positive film, the negative film, andthe separator comprises the bonding agent according to the presentdisclosure. The positive film comprises a positive electrode currentcollector and a positive electrode active substance layer. In oneaspect, the percent by weight of the bonding agent in the positiveelectrode active substance layer is 1.0 to 5.0%. In a further aspect,the percent by weight of the bonding agent in the positive electrodeactive substance layer is 1.5 to 2.5%.

Examples 1 to 8

The examples provide a method of preparing a bonding agent PVdF-Ac for apositive film of a lithium-ion battery, as well as a positive film and alithium-ion battery fabricated with said bonding agent.

Wherein, PVdF-Ac is prepared by means of emulsion polymerization, andthe molar contents of monomers are listed in Table 1:

TABLE 1 (unit: molar percent) vinylidene methyl acrylate acrylic aciddifluoride ethylene monomer monomer Example 1 60 10 25 5 Example 2 62 1025 3 Example 3 64 10 25 1 Example 4 67 10 20 3 Example 5 72 10 15 3Example 6 65 5 25 5 Example 7 55 10 25 10 Example 8 64.9 10 25 0.1

The polymerization method is:

Add deionized water, a dispersing agent, a pH adjusting agent, and achain transferring agent; remove oxygen in vacuum; add a fixed amount ofacrylate, acrylic acid monomer, and ½ of the above desired amount ofvinylidene difluoride; add an initiator; control temperature andpressure; start the polymerization reaction; and continuously add theremaining ½ of vinylidene difluoride, and ethylene; and when thepolymerization ends, subject the polymer to demulsification, washing,and drying, and obtain a PVdF-Ac product with a molecular weight of600,000 to 1.20 million.

Examples 1 to 8 provide lithium-ion batteries, comprising a positivefilm, a negative film, a separator, and an electrolyte. The positivefilm comprises a positive electrode current collector and a positiveelectrode active substance layer. The positive electrode currentcollector may be aluminum foil. The positive electrode active substancelayer comprises the following ingredients according to percent byweight:

The positive electrode active substance is LiCoO₂ with a content of95.5%;

The positive electrode bonding agent is PVdF-Ac obtained in Examples 1to 8, respectively, with a molecular weight of 600,000 to 1.20 millionand a content of 2.5%;

The content of the positive electrode conductive agent is 2.0%;

The positive electrode current collector is aluminum foil with athickness of 14 μm.

The negative film comprises a negative electrode current collector and anegative electrode active substance layer. The negative electrode activesubstance layer comprises the following ingredients according to percentby weight:

The negative electrode active substance is artificial graphite with acontent of 95.0%;

The negative electrode bonding agent is SBR with a content of 2.0%;

The negative electrode paste stabilizing agent is sodium carboxymethylcellulose with a content of 2%;

The content of the negative electrode conductive agent is 1.0%;

The negative electrode current collector is copper foil with a thicknessof 10 μm.

The separator is a polyethylene separator with a thickness of 14 μm.

The electrolyte comprises an organic solvent and a lithium salt. Theorganic solvent is a mixture of diethyl carbonate, dimethyl carbonate,and ethylene carbonate. The volumetric ratio of the three solvents is1:1:1. The lithium salt is LiPF₆ with a concentration of 1 mol/L.

The method for fabricating a lithium-ion battery is:

Preparation of a positive film: add 95.0% LiCoO₂, 3.0% PVdF-Ac, and 2.0%positive electrode conductive agent into NMP; mix homogeneously; coatonto aluminum foil; and obtain a positive film through drying, rolling,cutting, and welding a positive electrode tab;

Preparation of a negative film: add 95.0% artificial graphite, 2.0%sodium carboxymethyl cellulose, 1.0% negative electrode conductiveagent, and 2.0% SBR into distilled water; mix homogeneously; coat ontocopper foil; and obtain a negative film through drying, rolling,cutting, and welding a negative electrode tab;

Preparation of a battery: wind the positive film, the negative film, andthe separator into a battery core; place the battery core in an aluminumlaminated film; bake to remove water; then inject an electrolyte toperform formation and aging on the battery core; and obtaincorresponding lithium-ion batteries B1 to B8.

Comparison Examples 1 to 3

Prepare lithium-ion batteries in Comparison Examples 1 to 3 accordinglyto the method described above for Examples 1 to 8. The difference is inthat the added positive electrode bonding agents are different forComparison Examples 1 to 3. The compositions of the positive electrodebonding agents in Comparison Examples 1 to 3 are specifically listed inTable 2 below.

The fabrication processes for the positive films, the negative films,and the batteries are the same as those for Examples 1 to 8, and theobtained batteries are Bd1, Bd2, and Bd3, respectively.

TABLE 2 Positive electrode bonding agent Comparison homopolymerpolyvinylidene fluoride with a molecular Example 1 weight of 600,000 to1.20 million and a content of 2.5% Comparison copolymer PVdF-HFP with amolecular weight of Example 2 600,000 to 1.20 million and a content of2.5% Comparison A blend of PVdF and polymethyl acrylate: PVdF havingExample 3 a molecular weight of 600,000 to 1.20 million and a content of2.0%, and polyacrylic acid having a molecular weight of 200,000 to 1million and a content of 0.5%

Comparison Examples 4 to 10

Prepare lithium-ion batteries in Comparison Examples 4 to 10 accordinglyto the method described above for Examples 1 to 8. The difference isthat the proportions of monomers added into the positive electrodebonding agents PVdF-Ac are different. The proportions of 4 monomers inPVdF-Ac of Comparison Examples 4 to 10 are specifically listed in Table3 below.

The fabrication processes for the positive films, the negative films,and the batteries are the same as those for Examples 1 to 8, and theobtained batteries are Bd4, Bd5, Bd6, Bd7, Bd8, Bd9, and Bd10,respectively.

TABLE 3 (unit: molar percent) vinylidene methyl acrylate acrylic aciddifluoride ethylene monomer monomer Comparison 90 — 10 — Example 4Comparison 95 — — 5 Example 5 Comparison 85 — 10 5 Example 6 Comparison80 20 — — Example 7 Comparison 85  5 10 — Example 8 Comparison 75 20 — 5Example 9 Comparison 40 20 20 20  Example 10

(I) Stress-Strain Test on Bonding Agent Adhesive Films:

Use PVdF-Ac in Example 4 to prepare a 10% by weight NMP solution; takecertain amount of the solution and place it into a mold; dry at 80° C.in an oven; and obtain an adhesive film of PVdF-Ac. Correspondingly,prepare an adhesive film of homopolymer PVdF. Take adhesive films ofPVdF-Ac and homopolymer PVdF with the same width and thickness, teststress-strain curves, which can provide breaking strength and elongationat break. FIG. 1 is a diagram 100 illustrating comparison curves ofbreaking strength and elongation at break of adhesive films of PVdF-Acand homopolymer PVdF.

(II) Flexibility of Positive Films:

Use homopolymer PVdF or copolymer PVdF-Ac for comparison as a positiveelectrode bonding agent, and prepare a positive film according to thesteps described above for Examples 1 to 8. Take positive films under thesame compact density; use a paint film flexibility tester to test theflexibility of the electrode films. The curvature radius of the mandrelrod used is 0.5 mm. The difference in flexibility between the electrodefilms can be observed, as shown in FIG. 2 and FIG. 3. FIG. 2 is adiagram 200 illustrating a photo of an electrode film with homopolymerPVdF as a bonding agent. When a paint film flexibility tester (thecurvature radius of the mandrel rod used is 0.5 mm) is used to test theelectrode film with homopolymer PVdF as a bonding agent, some areas arebrittle and broken (with light passing through) after the electrode filmis bent. Correspondingly, FIG. 3 is a diagram 300 illustrating anelectrode film with copolymer PVdF-Ac as a bonding agent. When theelectrode film with copolymer PVdF-Ac as a bonding agent is tested by apaint film flexibility tester (the curvature radius of the mandrel rodused is 0.5 mm), there are no issues such as brittle and broken or lightpassing after the electrode film is bent. Namely, the electrode filmwith copolymer PVdF-Ac as a bonding agent has a better flexibility thanthe electrode film with homopolymer PVdF as a bonding agent does.

(III) Bonding Force Test on Positive Films:

Before immersion in an electrolyte: take a positive film after coatingand cold pressing, and cut into 100 mm long and 10 mm wide rectangles.Take a 25 mm wide stainless steel panel, attach a piece of double facedadhesive tape (11 mm wide) thereto, attach the cut electrode film ontothe double faced adhesive tape on the stainless steel panel, use a 2000g press roll to roll back and forth on the surface of the cut electrodefilm for 3 times (300 mm/min). Bend the electrode film by 180 degrees,manually rip apart 25 mm, fix said specimen onto a tester such that thepeeling surface is aligned with the force line of the tester, the testercontinuously peels off at 300 mm/min, obtain a peeling force curve, takethe average of the smooth segment as the peeling force F₀. Then thebonding force of a positive film being tested is: F=F₀/0.01=100F₀ (N/m).

After immersion in an electrolyte: take a positive film after coatingand cold, and cut into 100 mm long and 10 mm wide rectangles. Immerse inan electrolyte, the electrolyte comprises an organic solvent and alithium salt. The organic solvent is a mixture of diethyl carbonate,dimethyl carbonate, and ethylene carbonate. The volumetric ratio of thethree solvents is 1:1:1. The lithium salt is LiPF₆, with a concentrationof 1 mol/L. Place in a dry environment (with the relative humidity <5%);immerse at room temperature for 24 h; evaporate naturally in a dryenvironment; and when the solvent is completely evaporated from thesurface of the positive film, test the bonding force of the positivefilm. The testing method and bonding force calculation method are thesame as above. See Table 4 below for the testing data.

TABLE 4 Bonding force data of positive films (Unit: N/m) Before Afterimmersion in immersion in an electrolyte an electrolyte Example 1 182.3122.7 Example 2 138.1 104.3 Example 3 134.9 93.4 Example 4 122.4 99.1Example 5 106.8 86.5 Example 6 165.3 109.7 Example 7 211.8 156.2 Example8 85.2 18.3 Comparison Example 1 21.5 4.8 Comparison Example 2 17.7 2.1Comparison Example 3 81.7 2.5 Comparison Example 4 72.5 6.3 ComparisonExample 5 113.2 49.6 Comparison Example 6 131.9 64.3 Comparison Example7 24.3 3.5 Comparison Example 8 73.7 5.9 Comparison Example 9 117.8 53.8Comparison Example 10 483.3 282.6

(IV) Battery Performance Test:

1. Battery Discharge Rate Performance Test

1) At normal temperature, charge to 4.35 V at a constant current of 0.5C (C-rate), and then charge at a constant voltage until 0.05 C.Discharge to 3.0 V at a constant current of 0.5 C, record capacity, anduse this capacity as 100%;

2) At normal temperature, charge to 4.35 V at a constant current of 1.0C, and then charge at a constant voltage until 0.05 C. Discharge to 3.0V at a constant current of 0.5 C, record capacity, and calculatepercent;

3) At normal temperature, charge to 4.35 V at a constant current of 1.5C, and then charge at a constant voltage until 0.05 C. Discharge to 3.0V at a constant current of 0.5 C, record capacity, and calculatepercent;

4) At normal temperature, charge to 4.35 V at a constant current of 2.0C, and then charge at a constant voltage until 0.05 C. Discharge to 3.0V at a constant current of 0.5 C, record capacity, and calculatepercent.

See Table 5 for the testing data.

TABLE 5 Battery discharge rate performance (Unit: %) 0.5 C 1.0 C 2.0 C3.0 C Example 1 100.0 93.6 80.7 56.3 Example 2 100.0 93.8 81.9 58.8Example 3 100.0 94.5 84.3 69.2 Example 4 100.0 93.9 83.5 62.3 Example 5100.0 93.3 81.6 58.7 Example 6 100.0 92.7 82.5 67.5 Example 7 100.0 91.977.6 51.2 Example 8 100.0 95.1 86.5 69.9 Comparison Example 1 100.0 92.978.3 52.3 Comparison Example 2 100.0 92.6 77.1 49.9 Comparison Example 3100.0 91.7 71.1 31.9 Comparison Example 4 100.0 93.4 79.9 56.4Comparison Example 5 100.0 91.3 76.8 49.1 Comparison Example 6 100.093.6 78.9 55.0 Comparison Example 7 100.0 92.7 77.6 51.1 ComparisonExample 8 100.0 93.1 78.3 54.7 Comparison Example 9 100.0 91.7 76.6 50.8Comparison Example 10 100.0 88.3 70.6 42.5

2. Battery Cycle Life Performance Test

1) At normal temperature, charge to 4.35 V at a constant current of 0.5C, and then charge at a constant voltage until 0.05 C.

2) Discharge to 3.0 V at a constant current of 0.5 C, record capacity,and use the first recorded battery capacity as 100%;

3) Repeat Steps 1 and 2, and record the percent of remaining batterycapacity.

See Table 6 for the testing data.

TABLE 6 Battery cycle life performance (Unit:%) 0 100 200 300 400Example 1 100.0 92.2 91.6 87.3 81.9 Example 2 100.0 93.8 93.0 89.7 85.3Example 3 100.0 94.1 92.9 90.1 86.1 Example 4 100.0 94.0 93.1 90.9 86.9Example 5 100.0 93.8 92.8 89.9 85.8 Example 6 100.0 93.5 92.7 89.5 85.1Example 7 100.0 91.8 90.6 87.9 82.6 Example 8 100.0 93.9 92.6 89.6 84.8Comparison Example 1 100.0 91.9 88.1 82.7 76.9 Comparison Example 2100.0 92.9 90.3 85.1 77.6 Comparison Example 3 100.0 91.0 86.9 61.3 31.7Comparison Example 4 100.0 94.1 91.2 86.2 75.2 Comparison Example 5100.0 91.2 85.3 79.1 72.3 Comparison Example 6 100.0 93.5 91.0 86.6 79.2Comparison Example 7 100.0 91.3 87.2 80.3 69.4 Comparison Example 8100.0 92.1 88.6 82.0 71.5 Comparison Example 9 100.0 92.7 91.1 85.3 78.1Comparison Example 10 100.0 90.3 85.6 77.0 59.6

Examples 9 to 22

Prepare a bonding agent according to the method in Example 1. Thestructural formula of the polymer is shown by Formula V, whereinsubstituting groups of R₁ to R₁₁ are listed in Table 7, and values of a,b, c and d are listed in Table 8:

TABLE 7 Example R₁ R₂ R₃ R₄ R₅ R₆ R₇ R₈ R₉ R₁₀ R₁₁ Example 9 —H —H —H —H—H —H —H —CH₃ —CF₃ —H —H Example 10 —CH₃ —H —H —H —H —H —CH₃ —CH₂CH₃—CF₃ —H —H Example 11 —CH₃ —CH₃ —H —H —H —H —CH₃ —CH₃ —H —H —CH₃ Example12 —H —H —CH₃ —H —H —H —CH₃ n-butyl —H —H —CH₃ Example 13 —H —H —CH₃—CH₃ —H —H —CH₃ isobutyl —H —H —H Example 14 —H —H —H —H —CH₃ —H —Ht-butyl —H —H —H Example 15 —CH₃ —H —H —H —CH₃ —H —H 2-ethylhexyl —H—CH₃ —H Example 16 —CH₃ —H —H —H —CH₃ —H —H C₁₀ n-alkyl —H —CH₃ —HExample 17 —H —H —H —H —CF₃ —H —H n-butyl —CH₃ —H —H Example 18 —H —H —H—H —CF₃ —H —H n-propyl —CH₃ —H —H Example 19 —H —H —H —H —CF₃ —H —Hcyclohexyl —H —H —CF₃ Example 20 —H —H —H —H —H —H —CH₃ trifluoroethyl—H —H —CF₃ Example 21 —CH₃ —H —H —H —H —H —CF₃ isobornyl —CF₃ —H —CF₃Example 22 —CH₃ —H —H —H —H —H —CF₃ C₁₂ n-alkyl —CF₃ —H —H

TABLE 8 (unit: molar percent) a b c d Example 9 50 10 25 5 Example 10 900.1 9.8 0.1 Example 11 75 1 23 1 Example 12 80 10 7 3 Example 13 80 10 82 Example 14 55 15 25 5 Example 15 60 15 20 5 Example 16 65 10 20 5Example 17 70 8 12 10 Example 18 68 12 15 5 Example 19 73 15 10 2Example 20 70 10 14 6 Example 21 75 1 14 10 Example 22 50 20 20 10

Lithium-ion batteries fabricated with bonding agents having structuralformulae in Examples 9 to 22 have a similar performance to those in thepreceding examples.

The present disclosure has been disclosed with reference to preferredexamples as above, which, however, are not used to limit the claims.without departing from the concept of the present disclosure, thoseskilled in the art may make some possible variations and modifications.Therefore, the scope of the present disclosure shall be subject to thescope defined by the claims of the present disclosure.

According to the disclosure and description above, those skilled in theart may further make variations and modifications to the aboveembodiments. Therefore, the present disclosure is not limited by thespecific embodiments disclosed and described above. Some equivalentvariations and modifications to the present disclosure shall also beencompassed the claims of the present disclosure. Although theDescription uses some specific terms, in addition, the terms are usedonly for the purpose of easy description, which do not constitute anylimitation to the present disclosure.

What is claimed is:
 1. A bonding agent, the bonding agent being apolymer comprising: structural units represented by Formula I, FormulaII, Formula III, and Formula IV:

wherein each of R₁, R₂, R₃, and R₄ is independently selected from thegroup consisting of hydrogen, and C₁₋₈ straight-chain or branched alkylgroups substituted by a substituting group or not substituted by asubstituting group, wherein each of R₅, R₆, and R₇ is independentlyselected from the group consisting of hydrogen, and C₁₋₆ straight-chainor branched alkyl groups substituted by a substituting group or notsubstituted by a substituting group, wherein R₈ is selected from C₁₋₁₅alkyl groups substituted by a substituting group or not substituted by asubstituting group, wherein each of R₉, R₁₀, and R₁₁ is independentlyselected from the group consisting of hydrogen, and C₁₋₆ straight-chainor branched alkyl groups substituted by a substituting group or notsubstituted by a substituting group, wherein the substituting group isselected from halogens, and wherein each of n₁, n₂, n₃ and n₄ isindependently an integer greater than
 0. 2. The bonding agent of claim1, wherein: a molar percent of structural units represented by Formula Iin the bonding agent is 50% to 90%; a molar percent of structural unitsrepresented by Formula II in the bonding agent is 0.1% to 20%; a molarpercent of structural units represented by Formula III in the bondingagent is 1% to 25%; a molar percent of structural units represented byFormula IV in the bonding agent is 0.1% to 10%.
 3. The bonding agent ofclaim 2, wherein: the molar percent of structural units represented byFormula I in the bonding agent is 60% to 75%; the molar percent ofstructural units represented by Formula II in the bonding agent is 5% to10%; the molar percent of structural units represented by Formula III inthe bonding agent is 10% to 25%; the molar percent of structural unitsrepresented by Formula IV in the bonding agent is 3% to 5%.
 4. Thebonding agent of claim 1, wherein, in Formula II, each of R₁, R₂, R₃,and R₄ is independently selected from the group consisting of hydrogen,and C₁₋₆ straight-chain or branched alkyl groups.
 5. The bonding agentof claim 4, wherein, in Formula II, each of R₁, R₂, R₃, and R₄ isindependently selected from the group consisting of hydrogen and C₁₋₃alkyl groups.
 6. The bonding agent of claim 1, wherein, in Formula III,each of R₅, R₆, and R₇ is independently selected from the groupconsisting of hydrogen and C₁₋₃ alkyl groups, wherein R₈ is selectedfrom C₁₋₁₂ alkyl groups substituted by a substituting group or notsubstituted by a substituting group.
 7. The bonding agent of claim 6,wherein, in Formula III, each of R₅, R₆, and R₇ is independentlyselected from the group consisting of hydrogen, methyl, andtrifluoromethyl, wherein R₈ is selected from the group consisting ofmethyl, ethyl, n-butyl, isobutyl, sec-butyl, t-butyl, n-propyl,cyclohexyl, C₁₂ alkyl, 2-ethylhexyl, isobornyl, trifluoroethyl, andtrifluoromethyl.
 8. The bonding agent of claim 1, wherein, in FormulaIV, each of R₉, R₁₀, and R₁₁ is independently selected from the groupconsisting of hydrogen and C₁₋₃ alkyl groups.
 9. The bonding agent ofclaim 8, wherein, in Formula IV, each of R₉, R₁₀, and R₁₁ isindependently selected from the group consisting of hydrogen, methyl,and trifluoromethyl.
 10. The bonding agent of claim 1, wherein a numberaverage molecular weight of the bonding agent is 500,000 to 1.2 million.11. A lithium-ion battery, comprising: a positive film; a negative film;a separator; and an electrolyte, wherein at least one of the positivefilm, the negative film, and the separator comprises a bonding agent,the bonding agent being a polymer comprising: structural unitsrepresented by Formula I, Formula II, Formula III, and Formula IV:

wherein each of R₁, R₂, R₃, and R₄ is independently selected from thegroup consisting of hydrogen, and C₁₋₈ straight-chain or branched alkylgroups substituted by a substituting group or not substituted by asubstituting group, wherein each of R₅, R₆, and R₇ is independentlyselected from the group consisting of hydrogen, and C₁₋₆ straight-chainor branched alkyl groups substituted by a substituting group or notsubstituted by a substituting group, wherein R₈ is selected from C₁₋₁₅alkyl groups substituted by a substituting group or not substituted by asubstituting group, wherein each of R₉, R₁₀, and R₁₁ is independentlyselected from the group consisting of hydrogen, and C₁₋₆ straight-chainor branched alkyl groups substituted by a substituting group or notsubstituted by a substituting group, wherein the substituting group isselected from halogens, and wherein each of n₁, n₂, n₃ and n₄ isindependently an integer greater than
 0. 12. The lithium-ion battery ofclaim 11, wherein: a molar percent of structural units represented byFormula I in the bonding agent is 50% to 90%; a molar percent ofstructural units represented by Formula II in the bonding agent is 0.1%to 20%; a molar percent of structural units represented by Formula IIIin the bonding agent is 1% to 25%; a molar percent of structural unitsrepresented by Formula IV in the bonding agent is 0.1% to 10%.
 13. Thelithium-ion battery of claim 12, wherein: the molar percent ofstructural units represented by Formula I in the bonding agent is 60% to75%; the molar percent of structural units represented by Formula II inthe bonding agent is 5% to 10%; the molar percent of structural unitsrepresented by Formula III in the bonding agent is 10% to 25%; the molarpercent of structural units represented by Formula IV in the bondingagent is 3% to 5%.
 14. The lithium-ion battery of claim 11, wherein, inFormula II, each of R₁, R₂, R₃, and R₄ is independently selected fromthe group consisting of hydrogen, and C₁₋₆ straight-chain or branchedalkyl groups.
 15. The lithium-ion battery of claim 14, wherein, inFormula II, each of R₁, R₂, R₃, and R₄ is independently selected fromthe group consisting of hydrogen and C₁₋₃ alkyl groups.
 16. Thelithium-ion battery of claim 11, wherein, in Formula III, each of R₅,R₆, and R₇ is independently selected from the group consisting ofhydrogen and C₁₋₃ alkyl groups, wherein R₈ is selected from C₁₋₁₂ alkylgroups substituted by a substituting group or not substituted by asubstituting group.
 17. The lithium-ion battery of claim 11, wherein, inFormula IV, each of R₉, R₁₀, and R₁₁ is independently selected from thegroup consisting of hydrogen and C₁₋₃ alkyl groups.
 18. The lithium-ionbattery of claim 11, wherein a number average molecular weight of thebonding agent is 500,000 to 1.2 million.
 19. The lithium-ion battery ofclaim 11, wherein the lithium-ion battery comprises the bonding agent inthe positive film.
 20. The lithium-ion battery of claim 19, wherein thepositive film comprises a positive electrode current collector and apositive electrode active substance layer, and percent by weight of thebonding agent in the positive electrode active substance layer is 1.0%to 5.0%.